Clarify BUNDLE_PATH documentation.

* BUNDLE_PATH is used for installing AND finding gems
* When set
  * GEM_HOME and GEM_PATH are ignored
  * All gems are installed and retrieved from the BUNDLE_PATH location

[skip ci]

man/bundle-config.ronn

@@ -84,8 +84,10 @@ The following is a list of all configuration keys and their purpose. You can
 learn more about their operation in [bundle install(1)][bundle-install].
 
 * `path` (`BUNDLE_PATH`):
-  The location on disk to install gems. Defaults to `$GEM_HOME` in development
-  and `vendor/bundle` when `--deployment` is used
+  The location on disk where all gems in your bundle will be located regardless
+  of `$GEM_HOME` or `$GEM_PATH` values. Bundle gems not found in this location
+  will be installed by bundle install. Defaults to `Gem.dir`. When --deployment is
+  used, defaults to vendor/bundle.
 * `frozen` (`BUNDLE_FROZEN`):
   Disallow changes to the `Gemfile`. Defaults to `true` when `--deployment`
   is used.